The present invention relates generally to equipment for placing cables in ducts and more particularly to a portable cable placing system for pulling fiber optic cable through duct work without exceeding the cable's tensile strength. The invention is capable of handling theoretically endless lengths of cable using a plurality of appropriately spaced, individually powered puller devices so that the need to splice shorter lengths of cable is eliminated.
Fiber optic cable has been heralded as the telephone and data communication transmission line of the future. Among the advantages of fiber optic transmission cable is its extremely wide bandwidth. The wide bandwidth allows far more communication channels than metallic conductors, and more reliable computer data transmissions at much higher baud rates. In addition, fiber optic cable is smaller and lighter than metallic cable.
There are, however, difficulties with fiber optic cable, which have heretofore prevented its widespread use. The individual fibers, which are bundled to comprise a fiber optic cable, are not much more than the thickness of a human hair, which makes them very difficult to splice together. Splicing two ends of a fiber optic cable entails the tedious and expensive microscopic procedure of aligning the correct individual fibers and splicing them together. It costs about $16,000-$18,000 to splice a 144 fiber cable. Moreover, there is virtually always a signal loss at the splicing junction and this signal loss is particularly difficult to control due to the microscopic size of the fibers and the inability to make a perfectly aligned end-to-end connection. To overcome this signal loss, it is customary to use signal regeneration stations at periodic intervals. Each regeneration station can cost on the order of $350,000 to $400,000, hence, signal loss due to splicing is extremely expensive.
Many fiber optic cable systems are placed underground. Underground fiber optic cable systems typically comprise a network of underground ducts intercepted at strategic locations by manholes. Fiber optic cables often share existing duct work of older electrically conductive transmission cables. In many cases existing electrical cable ducts have been designed with a view towards later expansion. A typical duct comprises a plurality of tubelike passageways or conduits of plastic, metal or concrete extending parallel to one another and opening into manholes.
Placing the fiber optic cable in an underground duct has heretofore proven to be quite difficult due to the comparatively low tensile strength of fiber optic cable. Fiber optic cables are currently manufactured having tensile strengths of 300 pounds, 600 pounds and 1,000 pounds. By comparison, a copper cable has a tensile strength on the order of 5,000 pounds. Due to the fragile nature of fiber optic cable, it is not possible to pull a fiber optic cable through duct work using the techniques employed in pulling copper cable. Hence, the placing of fiber optic cable in underground ducts has heretofore been accomplished by a very labor intensive procedure. According to conventional procedures, the fiber optic cable is pulled using hydraulic powered pinch wheels, similar to conventional V-belt pulleys, which pinch the cable between the pulley groove. To ensure that the cable tensile limits are not exceeded, the prior art procedure requires at least one and usually two workers at each pulley station to feed the cable onto the pulley by hand while watching it carefully for signs of overtensioning. Aside from being labor intensive, the conventional procedure is slow and is difficult to implement when pulling long cable sections. The undesirability of conventional fiber optic cable pulling techniques may be summed up in terms of cost. The procedure is labor intensive, with large attendant labor costs, and places a practical limit on the length of cable sections which can be pulled without breakage, hence, splicing costs and regeneration station costs are also significant.
The present invention solves the aforementioned fiber optic cable placing problems by providing a collapsible and readily transportable cable pulling apparatus which synchronizes the cable pulling speed of several pullers acting in concert and thereby regulates the tension placed on the cable so that it will not break or become internally damaged. The invention provides a generally vertically arranged column having a plurality of legs extending laterally outwardly from and pivotally connected to the column. The legs are adapted for swinging movement in a horizontal plane between a relatively compactly folded position and an outwardly spread position of use. When folded, the cable puller is compact enough to fit through the opening of a manhole and can be easily managed by one or two persons. The cable pulling apparatus further comprises a motor driven cable pulling capstan carried on the column for rotation about a generally horizontal axis. The capstan hub is conical and has a diameter which varies in size along the axis to form a linear ramp or taper between the smaller diameter on one side and the larger diameter on the other side. The capstan is driven through a torque limiter which slips at a predefined limit torque to prevent breakage of the cable.
Further in accordance with the invention, there is provided a cable puller system comprising a plurality of cable pullers as described above which act in concert to coordinate the pulling speed in the various sections of the cable and to regulate the tension throughout the length of the cable being pulled. The individual cable pullers are each driven at substantially the same speed by self-contained motors. One cable puller might be placed at the entrance of a cable duct and a second placed at the exit of the duct. By wrapping the cable at least once around the capstan of each puller, the cable is pulled through the duct by driving forces supplied by the motorized capstans. Frictional forces between the cable and capstan, which are induced by tension in the destination side of the cable, cause the loop portion to ride up the conical incline as tension increases and to slip down the conical incline as tension decreases. The velocity of the cable as it leaves the capstan depends upon the diameter about which the cable is wrapped and the cable will thus ride and slip up and down the capstan until a speed regulating equilibrium is reached. A plurality of cable pullers can thus act in concert and maintain the proper cable speed and tension notwithstanding that sections of the cable may be experiencing different frictional loads due to differences in duct running length.
For a more complete understanding of the invention, its objects and its advantages, reference may be had to the following specification and to the accompanying drawings.